One component solution for meeting the rapidly increasing demand for data in future networks (a projected 1000 times increase in the coming decade) is believed to be a huge densification of a radio network's infrastructure. Essentially, by increasing the number of network access points per square meter, the average distance between an access point and a mobile node is reduced and, furthermore, it is likely that less user mobiles need to be served per access point. Both of these effects account for a potential increase of the average data rates per user, and thus a huge increase of the network's sum-rate.
FIG. 1 illustrates a general access network topology where nodes (network access points, APs and user equipment nodes, UEs) are connected by an edge if the signal to noise ratio would allow data communication. While FIG. 1a illustrates a general case, FIG. 1b and FIG. 1c show extreme cases with respect to the network's density. Historically, radio access networks have been of the sparse type as illustrated in FIG. 1b, i.e., the number of UEs greatly exceeds the number of APs. In contrast, future ultra-dense networks are believed to be better reflected by the network topology in FIG. 1c, i.e., the number of APs in a certain geographical region greatly exceeds the number of UEs.
It is not likely that future networks will be ultra-dense everywhere and all the time. Typically, for reasons of deployment costs for instance, the dense network scenario topology of FIG. 1c (more APs than UEs) will be relevant for hotspots (indoor environment) and urban scenarios where the necessary backhaul infrastructure solutions are affordable. In other geographical regions, the classical network topology of FIG. 1b (less APs than mobile users) will still be the most efficient solution.
In a radio network where many different independent radio devices wish to communicate with each other, simultaneously and geographically in each other's proximity, radio interference is inevitable. In cellular networks, for instance, typically a number of access points jointly constitute a radio access network to which a large number of mobile devices can simultaneously connect. Not only is there a potential interference when a number of mobile devices in each other's vicinity wish to communicate with the same access point (intra-cell interference), but in regions where multiple access points can be received, a user may experience inter-cell interference. Furthermore, in a mesh network where a large number of users can connect to any other user in the network directly, interference is also imminent if no proper measures are taken. For the sake of minimizing this interference, typically, radio networks employ means to manage this interference.
Recently, the increasing energy consumption of radio networks has become more of an (economic and environmental) concern to the network operators. The transmit power required to accomplish the network's data communication, hence not only should be optimized with respect to the above-mentioned interference and the spectral efficiency, but also with respect to the network's cost related to the energy consumption. Power should not be wasted.
The limited physical nature of time, radio spectrum, and transmit power, quantities that all play an important role in radio interference, have given relevance to the notion of a radio resource, a generic term used to describe any of these quantities. An improper management of these quantities creates interference. Management of time and frequency involves the assignment of time-slots and frequency bands to relevant radio nodes, along with a grant or instruction to transmit a (maximum) radio power.
Another relevant conceptual notion is the distinction between the data plane and the control plane. While the data plane is concerned with the processing, transmission and reception of the voice or video signal or the data packets, the control plane deals with all kinds of signalling messages that handle the bookkeeping in the communications: these signalling messages assure that the data packets are not lost, that they arrive in time, and with the proper radio characteristics, that can be read by the intended receiver, etc. For a good management of the spectral efficiency and the energy-efficiency, a well-designed control plane is essential in most of today's networks.
Future networks represented by a geographical and temporal mix of the conceptual topologies in FIGS. 1b and 1c come with a number of new challenges related to the spectral efficiency and the energy efficiency.
First, in general, when the average distance between a network's access point decreases more than the average power of these access points (FIG. 1c), interference is likely to become more of a problem and radio resources must be efficiently managed to reduce this interference. At the same time, these management protocols should be implemented without increasing the load of the network's control plane. Also, when the network density increases more than the user density, the network's power consumption will potentially increase significantly. Simply, there will be more access points per user and, non-linearity of the power consumption (there is always a certain fixed power consumed, regardless how little data will be transmitted) will in principle cause the power per transmitted bit also to increase. For these reasons it is desirable that radio resources in an ultra-dense network are managed in such a way that energy-consumption per transmitted bit is low and that interference is kept low.
Moreover, in the mixed networks described above, where certain regions are sparse and other regions are dense (and where these local density-characterization changes from time to time), there is a need to manage the radio resources in a way that adapts to the network density reigning at a certain time and place. In particular, it is a problem to match access points and mobiles and assign radio resources in such a way that interference, energy consumption or other objective function are optimized, and in such a way that it is tailored to both ultra-dense regions of a network (FIG. 1c) and sparse regions of the same network (FIG. 1b).
Historically, several high-level concepts have been deployed to allocate radio resources in a network.
Frequency Planning and Frequency Re-Use
One traditional way to manage these limited radio resources is by assigning a fixed frequency to a certain geographical region. In second generation systems a licensed frequency band was partitioned into 3 disjoint sub-bands (reuse factor 3) and each base station was assigned one of these sub-bands. The assignment was fixed and static (could only be changed by reconfiguring the base station). Prior to deployment of the network a careful planning of the sub-band assignment was carried out in order to assure that base stations in each other's vicinity did not use the same sub-bands hence interference among different access points was reduced. A drawback of this type of planning is that it is static in nature, not flexible, expensive and generally inefficient.
Radio Network Controller Node
In third generation systems, all base stations use the entire licensed frequency band (frequency re-use 1). Much of the interference control and resource management is done by means of power control of the dedicated assigned radio channels. In particular an (outer loop) power control is carried out between the radio network controller, the RNC (a central network node) and each mobile node. A drawback with this prior art is its inefficiency with respect to interference control.
Multi-User Scheduling
In fourth generation systems (LTE), all base stations still use the entire licensed frequency band (frequency re-use 1) and the concept of multi-user scheduling in a common radio channel was introduced. Each user reports the quality of the channel (e.g., the level of interference it experiences) and the base station determines based on this information which radio resource to assign to each user. In the related art LTE system, co-channel and non-co-channel access point can be deployed in the same geographical area as illustrated in FIG. 2. A drawback with this prior art is a lack of good coordination between base stations and its efficiency with respect to inter-cell interference.
Access-Point on/Off Activity Scheduling
Access-point on/off activity scheduling was recently proved to lead to substantial energy savings and spectral efficiency gain in LTE-compliant heterogeneous network deployments.
Inter-Cell Interference Coordination (ICIC)
Inter-cell interference coordination (ICIC) was introduced in LTE systems to handle mitigate interference at the cell-edge. Neighbouring base stations exchange information related to interference measurements and as well as subframes where a base station shall refrain from transmitting data and control channels, referred to as called almost blank subframes (ABS). Interference indicators from other subframes are used for power control and scheduling to mitigate the interference at the neighbouring cell, whereas a schedule of ABS in a neighbouring cell is used to schedule cell-edge users in protected subframes where the neighbouring cells are silent.